Wireless LANs (local area networks) are widely used in homes, offices and the like. As wireless LANs become widespread, a large number of access points (APs) are installed, which are normally powered on while in use even when no communication data is generated.
Since such “left-on” APs are not used in a majority of time, power is wasted.
In the field of sensor networking, approaches have been considered that use a wakeup receiver to wake up the receiving end of data only when communications are required.
Non-Patent Document 1 proposes an approach that uses 802.15.4 sensor motes that consume less power than wireless LAN cards to observe a wireless LAN channel of the same frequency and, when it detects radio waves from the transmitting source, wakes up the wireless LAN card of the terminal.
Patent Documents 1 to 3 and Non-Patent Document 2 propose approaches that use wakeup receivers that consume less power to wake up wireless LAN cards.
In each of Patent Documents 1 and 2 and Non-Patent Document 2, since the frequency of wakeup signals is different from the frequency for the wireless LAN, two antennas are required.    Patent Document 1: JP 2007-526655 A    Patent Document 2: WO 04/100503    Patent Document 3: U.S. Pat. Pub. No. 2007/0253468    Non-Patent Document 1: NiLesh Mishra, Kameswari Chebrolu, Bhaskaran Raman, Abhinav Pathak: Wakeon WLAN, WWW 2006    Non-Patent Document 2: Shigemi Ishida, Makoto Suzuki, Takashi Morito, Hiroyuki Morikawa: Multi-Step Wakeup Scheme for Low-Power-Listening Wireless Communication System, IEICE technical report, information networks, 107(525), p. 355-360, Feb. 28, 2008